Reed valves are often employed in applications where a fluid is intended to flow in one direction through a passage but not in the opposite direction, much like a check valve. Though automotive applications for reed valves are generally rare, reed valves are commonly used within the intake systems of two-stroke engines, such as those employed for chain saws and motorcycles. Reed valves generally consist of a support structure, such as a housing, containing an aperture which is opened and closed by a resilient member, or reed, attached to the support structure adjacent to the aperture. The support structure is situated within a duct or wall between two chambers, with the aperture serving as the passage therebetween.
Reed valves are operated by the flow of the air/fuel mixture through the passage containing the reed valve. Under certain operating conditions, the particular fluid serves to force the reed against the support structure and thereby close the aperture. Under reverse conditions, the fluid serves to force the reed away from the aperture to permit flow through the aperture. For example, when used in a fuel system, the vacuum created by the combustion chamber deflects the reed away from the aperture to permit the air/fuel mixture to enter the combustion chamber.
In engine applications such as fuel intake systems, the reed must not only be resistant to thermal and chemical attack from the fluids being controlled but must also have sufficient structural integrity to withstand numerous and rapid cycling. In terms of stress, the reed experiences a cantilever bending moment when forced away from the aperture. When forced against the support structure, the reed is generally deflected at its center, being supported at its periphery by the support structure. The forces involved can be significant, requiring the reed to be formed from a strong and durable material.
In the past, reeds have generally been formed from steel. However, steel reeds have two major disadvantages. The first disadvantage is the high density of steel, which results in a heavy reed with a low natural frequency. This yields a slower response to flow reversals and therefore a less effective check valve. While this disadvantage is applicable to both two-stroke and four-stroke applications, it is more serious for four-stroke engines. In two-stroke engines, reed valves are mounted on the crankcase. Crankcases provide a larger volume of air, reducing the, importance of the reed valve having a high natural frequency. However, in four-stroke engines, the trapped air volume between the poppet valve and the reed valve is much smaller, such that fast reed valve response is needed, requiring the reed valve to have a higher natural frequency.
The second major disadvantage is that any failure of a steel reed from fatigue or impact will result in fragments of steel in the intake system. When ingested by the engine, the steel fragments will cause catastrophic damage to the cylinder and pistons, requiring, at the very least, substantial repairs and more often complete replacement of the engine. In addition, such a failure will typically render the engine inoperable, leaving the vehicle stranded.
As a result of these significant shortcomings, polymer composite reeds have recently become common. Polymer composite reeds typically have a fiberglass fabric or weave encased in a thermoset polymer, such as an epoxy resin. As such, polymer composite reeds are significantly less dense than steel reeds. In addition, broken composite reeds can be readily ingested by the engine with no apparent damage. As a result, the failure of a composite reed typically will only result in a slightly rough running engine that is still very drivable. Furthermore, where a composite reed has failed, only the reed must be replaced instead of the entire engine.
Conventionally, the fiberglass mesh (110) is in the form of a "plain weave", which is illustrated in FIGS. 1 and 2. "Plain weave" is defined as a fabric in which each strand, composed of hundreds of individual fiberglass filaments which are twisted or plied together, passes over and under successive transverse strands, one strand at a time, in an alternating fashion. As can be seen in FIG. 1, the appearance of a plain weave fabric 110 is a repetitive pattern of alternating strands. In the plan view illustrated in FIG. 1 and cross-sectionally in FIG. 2, it can be seen that each visible strand running in one direction (such as the strands 114) is "surrounded" by strands (116) running in the transverse direction. The regions 118 denote the epoxy resin used to encase the weaved fabric 110. Plain weave fabrics are typically manufactured with a balanced construction, wherein the number and size of the strands running in one direction are approximately the same as those strands running in the transverse direction. This balanced construction, in combination with the plain weave, yields a final composite which has approximately equal mechanical properties in both directions of the weave.
Conventionally, the suitability of a particular polymer composite material for a composite reed is evaluated in terms of its "flexural modulus." Typically, a composite reed will be tested by flexing a test specimen at its center while being supported at two peripheral points, such as the test method described in ASTM D-790. The flexural modulus indicates the stress-versus-strain relationship of the polymer composite reed material, which serves as an indication of the ability of the reed to open and close under the pressure loading found in its working environment.
With renewed interest in reed valve applications for two-stroke and four-stroke engines in the automotive industry, reed valves are now being required to last significantly longer, corresponding to the typical minimum 100,000 mile durability requirement manufacturers impose for automobiles. As a result, reed valves used in automotive applications must survive many more cycles than previously required in conventional applications such as motorcycles and chain saws. Thus, while suitable for many applications, current polymer composite reeds formed from fiberglass-reinforced thermoset materials tend to be inadequate for automotive applications. A primary reason for this is the inadequate chemical resistance of conventional thermoset composite reeds to automotive fuels, especially methanol and gasoline blends. Another reason is the limited fracture toughness available from thermoset materials.
The flexural modulus of fiberglass-reinforced thermoset reeds is about 20 to about 28 GPa for a typical thickness of about 0.4 millimeters. While such reeds are suitable for conventional applications such as that within the motorcycle industry, they tend to be inadequate for automotive applications which require lighter and faster responding reeds. A lighter reed could be obtained if the thickness of the reed were reduced. However, the natural frequency of a reed, by which the speed of closing is usually rated, is proportional to its thickness according to the equation: EQU f.sub.n =kt(E/.rho.).sup.1/2
where f.sub.n is natural frequency, k is a constant for a fixed length cantilevered beam, t is the thickness of the reed, E is the flexural modulus and .rho. is the reed density. As a result, any reduction in thickness will result in a slower responding reed. In order to compensate for any reduction in thickness, there must be a corresponding increase in the reed's flexural modulus.
Thus, it would be desirable to provide a reed for a reed valve which is suitable for automotive applications in terms of performance capability as defined by the reed's thickness and flexural modulus, and in terms of structural integrity as defined by the reed material's fracture toughness, so as to be able to survive numerous engine cycles without failure.